High polymers used in the manufacture of films and fibers must sometimes be modified to achieve the fire-retardant properties required for particular applications. This is done by physically mixing a fire-retardant agent into the polymer or by reacting such agents with the polymers. Such agents can affect the burning properties of polymers by, for example, (1) interfering with the combustion reaction, (2) making the products of pyrolysis less flammable, (3) reducing the transfer of heat from the flame to the solid, or (4) reducing the rate of diffusion of pyrolysis products to the flame front. Additionally, they can affect the drip characteristics of polymers (their tendency when exposed to flame to form burning droplets that help spread the fire).
These flame retardant agents include phosphorus, compounds containing phosphorus, bromine or chlorine, compounds containing bromine or chlorine (including tetrahalobisphenol As), anitmony trioxide and mixtures of one or more of these. Among the more widely used agents are those containing bromine or chlorine. Although these are effective flame-retardant agents in many applications, their utility has heretofore often been limited by their limited thermal stability, at high processing temperatures (above 200.degree. C.), by poor drip characteristics and/or by other properties such as relatively low molecular weight and poor film-forming characteristics. Hence, it has often been necessary to blend these agents or, polymers containing them, with major amounts of other non-halogen containing polymers in an attempt to overcome these difficulties. Thus, for example, Japanese Pre-Patent publication No. 48-49832 discloses that relatively low molecular weight polyethers prepared from the reaction of tetrabromobisphenol A and a dibromoalkane containing from 2-4 carbon atoms having a degree of polymerization of from 2-10 and a melting point of less than 200.degree. C. are useful as flame-retardant agents in thermoplastic resins.